Controlling sound in non-Hermitian acoustic systems

Abstract

The concept of non-Hermitian physics has inspired numerous intriguing phenomena in classical wave systems. By introducing judicious arrangements of gain and loss media, non-Hermitian modulation in acoustic metamaterials can give rise to many extraordinary wave-matter interactions that cannot be achieved naturally, which have been verified theoretically and experimentally by a series of previous works. Here, we aim to review recent developments in this topic. First of all, we explain the basic concepts and mathematical tools to deal with non-Hermitian acoustics by studying some pedagogical examples. Then, we present some interesting works that demonstrate the superior abilities of non-Hermitian modulation in wave manipulation. Also, we pay attention to the study of topological systems with non-Hermitian modulation, with a special emphasis on nontrivial topological states induced by non-Hermiticity alone. Finally, we present an outlook on the potential directions and applications for future works. We hope this review can provide a better understanding to stimulate research on acoustics beyond the Hermitian regime.